1. Field of the Invention
A certain aspect of this disclosure relates to a pixel clock generating device and an image forming apparatus including the pixel clock generating device.
2. Description of the Related Art
FIG. 26 is a schematic diagram of a related-art image forming apparatus. The image forming apparatus of FIG. 26 is, for example, a laser printer or a digital copier that forms an image by an electrophotographic process. As shown in FIG. 26, a laser beam (scanning beam) emitted from a semiconductor laser unit 1009 is deflected by a rotating polygon mirror 1003, passes through a scanning lens 1002, and forms a beam spot on a photoconductor 1001. The photoconductor 1001 is scanned and exposed by the beam spot and as a result, an electrostatic latent image is formed. During this process, a photodetector 1004 detects the scanning beam for each line.
A phase-locked loop 1006 receives a clock signal from a clock generating circuit 1005, generates a phase-locked (or phase-synchronized) image clock signal (pixel clock signal) for each line based on an output signal from the photodetector 1004, and supplies the generated image clock signal to an image processing unit 1007 and a laser driving circuit 1008. A pixel clock signal is used, for example, to determine the timing for processing pixels, to control the timing for scanning a line, and to control light sources (laser units). The laser driving circuit 1008 controls the light emitting time of the semiconductor laser unit 1009 according to image data generated by the image processing unit 1007 and the phase-locked image clock signal generated by the phase-locked loop 1006 for each line and thereby controls the formation of an electrostatic latent image on the photoconductor 1001.
In a scanning optical system as described above, variation in scanning speed leads to irregularity in an image and degrades image quality. Particularly when forming a color image, variation in scanning speed causes positional errors of dots of different colors in the main scanning direction. This in turn causes color shift and reduces color reproducibility and image resolution. Therefore, to improve image quality, it is necessary to reduce the variation in scanning speed.
Major causes of variation in scanning speed (scanning speed errors) are described below.
(1) Variation in scanning speed (between scan lines) related to surfaces of polygon mirror
Variation in scanning speed may be caused by the variation in distance of the surfaces (reflecting surfaces) of a deflector such as a polygon mirror from its rotating shaft (i.e., decentering of the axis of a polygon mirror) and the variation in precision of the surfaces. This type of variation in scanning speed has a cycle of several lines (for example, the number of lines corresponding to the number of surfaces of a polygon mirror).
(2) Variation in Average Scanning Speed
The average scanning speed indicates an average of scanning speeds of the surfaces of a polygon mirror. Variation in the average scanning speed is caused by, for example, the variation in rotational speed of a polygon mirror and various changes in a scanning optical system caused by environmental changes in temperature, humidity, vibration, and so on. Also, variation in average scanning speed may be caused by chromatic aberration in a scanning optical system that occurs when the oscillation wavelength of a semiconductor laser, or a light source, changes due to, for example, a temperature change. This type of variation in scanning speed is moderate compared to other types.
In a multi-beam optical system that includes multiple light sources such as a semiconductor laser array and where multiple light beams are scanned by a common scanning optical system at the same time, variation in scanning speed may be caused by reasons as described below.
(3) Variation in Scanning Speed Related to Light Sources
Variation in scanning speed may occur due to chromatic aberration of a scanning optical system when light sources have different oscillation wavelengths. Here, when the oscillation wavelength differs from one light source to another, the variation in average scanning speed as described in (2) may differ depending on the light source. Also, the scanning speed of laser beams may vary depending on the mounting accuracy of light sources.
In an image forming apparatus (tandem image forming apparatus) including multiple photoconductors and scanning optical systems and configured to form a color image, the difference in scanning speed of the scanning optical systems greatly affects the image quality.
(4) Variation in Scanning Speed Related Scanning Optical Systems
Variation in scanning speed related to scanning optical systems may be caused by inaccurate production and assembly of parts of the scanning optical systems and deformation of the parts over time. Also, since a tandem image forming apparatus generally includes multiple light sources, the variation as described in (3) may also occur. In a tandem image forming apparatus, the average scanning speed may differ from one scanning optical system to another, and the variation in scanning speed as described in (1) and (2) may be observed in each of the scanning optical systems.
There is an image forming apparatus where some components are shared by multiple scanning optical systems. Even in this case, since there are multiple optical paths from the light sources to the photoconductors, the variation in scanning speed described in (4) may also occur.
There is a known method where a scanning speed error is corrected by changing the frequency of a pixel clock signal depending on the scanning speed. In this method, the frequency of an oscillator for generating a pixel clock signal is controlled (i.e., phase-locked loop (PLL) control) so that the count of cycles of the pixel clock signal between the start and end of scanning becomes a predetermined value.
However, the above method has a disadvantage as described below. In the above method, the frequency of a reference clock signal used for phase comparison corresponds to one line, and is therefore far lower than (one in several thousands to one in tens of thousands) that of a pixel clock to be generated. Therefore, it is difficult to achieve sufficient open loop gain of the phase-locked loop and to accurately control the frequency of the pixel clock signal. Also, since the clock frequency, is easily affected by disturbance, it is difficult to accurately generate a pixel clock signal. Further, since it is necessary to change the control voltage of a voltage-controlled oscillator (VCO) for each scan to correct the scanning speed error for each surface, it takes a long time before the clock frequency becomes stable.
There is another method for correcting a scanning speed error where the phase of a pixel clock signal is controlled based on a generated high frequency clock signal. In this method, the phase of a pixel clock signal is controlled so that the count of cycles of the high frequency clock signal between the start and end of scanning becomes a predetermined value.
The high frequency clock signal is accurately generated based on an accurate reference clock signal generated, for example, by a crystal oscillator. Using such an accurate high frequency clock signal for the phase control of a pixel clock signal makes it possible to accurately generate the pixel clock signal. However, to correct a scanning speed error by controlling the phase of a pixel clock signal, it is necessary to generate phase control data for one scan line. Also, to reduce local deviation caused by the phase change of the pixel clock signal and thereby to accurately generate the pixel clock signal, it is necessary to perform high-resolution phase control and as a result, the size of the phase control data becomes large. Accordingly, it is difficult to accurately generate such large phase control data at high speed and a very high-speed control circuit is necessary to perform real-time control. Also, to correct scanning speed errors for respective surfaces of a polygon mirror with the above method, it is necessary to generate the phase control data for each surface. Therefore, in this case, it is necessary to generate and store a far larger amount of phase control data. Further, inaccurate production and assembly of parts of a scanning optical system may cause the scanning speed to vary even during the scanning of one line.
(5) Nonlinear Errors
FIG. 27A is a graph showing exemplary nonlinear errors in scanning speed during the scanning of one line. In FIG. 27A, the horizontal axis indicates positions X in a scan line and the vertical axis indicates scanning speeds V(X) at the positions X. Also a dashed-dotted line Vavg indicates the average scanning speed during the scanning of one line. When the scanning speed varies as shown in FIG. 27A, deviations Δ from a desired value at a constant scanning speed are indicated by a solid line in FIG. 27B. The deviations Δ indicate positional errors of dots that cause degradation of image quality. In FIG. 27B, the dotted line indicates deviations Δ in a case where scanning is performed in a direction from a position X2 to a position X1 in FIG. 27A. As shown in FIGS. 27A and 275, when scanning is performed in both directions with a scanning optical system where positional errors of dots occur asymmetrically with respect to the center of scanning, color shift increases and image quality is greatly degraded.
Also, the degree and distribution of the deviations Δ may vary depending on the precision of each surface of a polygon mirror. Further, the degree and distribution of the deviations Δ may vary from one scanning optical system to another.
There is a method for correcting nonlinear errors in scanning speed by modulating the frequency of a pixel clock signal according to the position in a scan line. With this method, however, since the center frequency of a pixel clock signal is generated in a conventional manner, it is difficult to generate an accurate pixel clock signal and effectively correct the nonlinear errors in scanning speed. Thus, this method is not effective to improve image quality.
Japanese Laid-Open Patent Publication No. 2006-305780 discloses a pixel clock generating device for generating a pixel clock signal that can accurately correct scanning speed errors and nonlinear errors as described in (1) through (5) above.
The disclosed pixel clock generating device includes a high-frequency clock generating unit for generating a high-frequency clock signal, a first edge detection unit for detecting a first synchronization signal, a second edge detection signal for detecting a second synchronization signal, and a comparing unit that compares the time interval between the first synchronization signal and the second synchronization signal with a target value to determine the difference. The disclosed pixel clock generating unit also includes a frequency divider that generates a pixel clock signal by frequency-dividing the high-frequency clock signal generated by the high-frequency clock generating unit, and a frequency calculation unit that outputs a frequency specification signal specifying a pixel clock signal frequency based on the difference determined by the comparing unit to correct the pixel clock signal and thereby to correct an error in the scanning speed.
Meanwhile, Japanese Laid-Open Patent Publication No. 2005-92129 discloses a tandem image forming apparatus that can form an image at high speed with a small number of light sources.
FIG. 28 illustrates an optical scanning device of the image forming apparatus disclosed in JP2005-92129. In the disclosed optical scanning device, two photoconductor drums 157a and 157b are scanned alternately in a time-sharing manner with light beams emitted from light sources 901 and 901′ to form two electrostatic latent images. The disclosed optical scanning device also includes one laser driving circuit for each of the light sources 901 and 901′. Each of the two laser driving circuit modulates, for alternate lines, signals for forming the electrostatic latent images on the photoconductor drums 157a and 157b. 
Here, the scanning speeds of light beams emitted from one light source and scanning the photoconductor drums 157a and 157b may vary for the reasons described above. In the optical scanning device of FIG. 28, the light beam for scanning the photoconductor drum 157a is deflected by an upper polygon mirror 907a of an optical deflector 907, and the light beam for scanning the photoconductor drum 157b is deflected by a lower polygon mirror 907b of the optical deflector 907. Therefore, as described in (1) above, the scanning speed of the light beam varies from one surface to another depending on the degree of precision of the polygon mirror, and the degree of variation also differs between the polygon mirrors. Also, since the light beams reach the photoconductor drums 157a and 157b via different optical components, the variation in scanning speed related to scanning optical systems as described in (4) may occur. Further, nonlinear errors as described in (5) may occur. Such variation (or errors) in scanning speed may cause positional errors in the main scanning direction of dots constituting images formed on the photoconductor drums 157a and 157b, and the positional errors of dots may cause color shift and reduces the image quality such as color reproducibility and image resolution.
Here, it may be possible to combine the pixel clock generating device disclosed in JP2006-305780 with the tandem image forming apparatus disclosed in JP2005-92129. However, with a configuration where one light source is used to optically scan different optical scanning positions (or lines) in a time-sharing manner, the light source is driven based on one pixel clock signal (i.e., there is only one target to be controlled). Since the scanning speed errors and the nonlinear errors (and also target values) differ from one optical scanning position to another, it is difficult to accurately correct those errors with one pixel clock signal.